P[ossessive] A[djective]s in Slavonic, formed from nouns via suffixation, show unusual syntactic behavior. In Upper Sorbian, the form of attributive modifiers, relative pronouns, and personal pronouns can be controlled by the syntactic features of the noun underlying the PA. Control of attributive modifiers gives rise to phrases in which word structure and phrase structure do not match. The fact that the underlying noun is available for syntactic purposes suggests that PA formation is an inflectional process, while other factors (such as change of word-class membership) point just as clearly to a derivational process. It thus appears that any sharp differentiation between inflectional and derivational morphology must be abandoned. Data presented from all thirteen Slavonic languages, based on extensive work with native speakers, show that the control possibilities of the PA vary considerably. However, control of the attributive modifier is possible only if control of the relative pronoun is also possible, and that in turn only if control of the personal pronoun is possible. This result is subsumed under the constraints of the Agreement Hierarchy.